(01 of13)
Open Image ModalSwat team members secure the scene near Sparks Middle School in Sparks, Nev., after a shooting there on Monday, Oct. 21, 2013. Authorities are reporting that two people were killed and two wounded at the Nevada middle school. (AP Photo/Kevin Clifford) (credit:AP)
Sandy Hook Elementary(02 of13)
Open Image ModalMolly Delaney, left, holds her 11-year-old daughter, Milly Delaney, during a service in honor of the victims who died a day earlier when a gunman opened fire at Sandy Hook Elementary School in Newtown, Conn., as people gathered at St. John\'s Episcopal Church , Saturday, Dec. 15, 2012, in the Sandy Hook village of Newtown, Conn. The massacre of 26 children and adults at Sandy Hook Elementary school elicited horror and soul-searching around the world even as it raised more basic questions about why the gunman, 20-year-old Adam Lanza, would have been driven to such a crime and how he chose his victims. (credit:AP Photo/Julio Cortez)
(03 of13)
Open Image ModalPolice secure the scene near Sparks Middle School after a shooting in Sparks, Nev., on Monday, Oct. 21, 2013. Authorities are reporting that two people were killed and two wounded at the Nevada middle school. (AP Photo/Kevin Clifford) (credit:AP)
Clackamas Town Center(04 of13)
Open Image ModalA security guard looks over the food court at the Clackamas Town Center mall as it opens, on Friday, Dec 14, 2012 in Portland, Ore. The mall is reopening, three days after a gunman killed two people and wounded a third amid a holiday shopping crowd estimated at 10,000. The shooter, Jacob Tyler Roberts, killed himself after the attack Tuesday afternoon. (credit:AP Photo/The Oregonian, Thomas Boyd)
St. Vincent's Hospital Shooting(05 of13)
Open Image ModalBirmingham police arrive at the scene of a shooting at St. Vincent\'s Hospital on Saturday, Dec. 15, 2012 in Birmingham, Ala. Authorities in Alabama say a man opened fire the hospital, wounding an officer and two employees before he was fatally shot by police. Birmingham Police Sgt. Johnny Williams says the officer and employees suffered injuries that are not considered life-threatening. (credit:AP Photo/al.com, Joe Songer)
Sikh Temple Shooting(06 of13)
Open Image ModalMourners attend the funeral and memorial service for the six victims of the Sikh temple of Wisconsin mass shooting in Oak Creek, Wis., Friday, Aug 10, 2012. The public service was held in the Oak Creek High School. Three other people were wounded in the shooting last Sunday at the temple. Wade Michael Page, 40, killed five men and one woman, and injured two other men. Authorities say Page then ambushed the first police officer who responded, shooting him nine times and leaving him in critical condition. A second officer then shot Page in the stomach, and Page took his own life with a shot to the head. (AP Photo/Jeffrey Phelps) (credit:AP)
July 2012: Aurora, Colorado(07 of13)
Open Image ModalA policeman stands outside a Century 16 movie theater in Aurora, Colo., where a heavily armed man opened fire, killing at least 12 people and injuring 50 others. (credit:AP)
May 2012: Seattle, Wash.(08 of13)
Open Image ModalFriends, family and employees react after a shooting at Cafe Racer in Seattle on May 30, 2012. A lone gunman killed four people Wednesday -- three were shot to death at a cafe, and a fourth in a carjacking. The gunman later killed himself. (credit:AP Photo/seattlepi.com, Joshua Trujillo)
April 2012: Oakland, California(09 of13)
Open Image ModalAlameda County Community Food Bank workers move a memorial from a parking spot next to Oikos University in Oakland, Calif., Monday, April 23, 2012. Some students and staff members have arrived to resume class at Oikus University, the small California Christian college where seven people were shot to death earlier in April. (credit:AP Photo/Jeff Chiu)
November 2009: Fort Hood, Texas(10 of13)
Open Image ModalPanou Xiong, center, is comforted by family and friends following a Remembrance Ceremony commemorating the one-year anniversary of the worst mass shooting on a U.S. military base, where 13 people were killed and dozens wounded,, Nov. 5, 2010 in Fort Hood, Texas. Xiong\'s son, Pfc. Kham Xiong, was killed in the shooting. \r\n\r\nCORRECTION: This slide originally said that the Fort Hood shooting took place in November 2010. The shooting took place in November 2009. (credit:AP Photo/Eric Gay)
March 2009: Kinston, Alabama(11 of13)
Open Image ModalThe charred Kinston, Ala. living room where suspected gunman Michael McLendon allegedly killed his mother Lisa McLendon, is photographed Wednesday, March 11, 2009. Authorities were working Wednesday to learn why a gunman set off on a rampage, killing 10 people across two rural Alabama counties. (credit:AP Photo/Dave Martin)
August 2007: Blacksburg, Va.(12 of13)
Open Image ModalAn unidentified family member of slain Virginia Tech student Daniel Alejandro Prez Cueva, pauses at his memorial stone after the dedication of the memorial for the victims of the Virginia Tech shooting in Blacksburg, Va., Sunday, Aug. 19, 2007. More than 10,000 people gathered on the main campus lawn as Virginia Tech dedicated 32 memorial stones for those killed by a student in a mass shooting on campus last April. (credit:AP Photo/Steve Helber)
April 1999: Littleton, Colo.(13 of13)
Open Image ModalThis aerial shows the news media compound near Columbine High School in Littleton, Colo., April 21, 1999. Media from around the world poured into the area after 15 people were killed during a shooting spree inside the school. (credit:AP Photo/Ed Andrieski)